By July 1942 the days of easy pickings for the U-boats along the Eastern
Seaboard of the USA were over. It was time for Donitz to switch his efforts
back to his old hunting grounds in the mid-Atlantic. He had seemingly ample
grounds for optimism. In May 1942 German experts had produced a study which
concluded that if the U-boats were able to sink a monthly average of 700,000
tons of Allied merchant ships for the rest of the year, Britain, despite all
the efforts of shipbuilding yards on both sides of the Atlantic, would be
doomed.

Though this was more than twice the average monthly sinkings for 1941, Donitz
felt confident of success. At the end of 1941, his U-boat fleet had totalled
236 vessels, which had been sinking 13 Allied merchant ships for every one of
their own number lost. They had reduced the total available British merchant
fleet by 3 million tons compared with the start of the war. And now Donitz was
returning to the Atlantic convoy routes in a stronger position. He had a total
U-boat strength of 331, of which 141 were operational and an average of 50
constantly on patrol. U-boat HQ at Chateau Kernival in Brittany had become
expert in the close orchestration of the increasingly effective "wolf-pack"
tactics.

Even more significantly, and often overlooked in favour of the better-known
Allied successes in breaking the Enigma codes, the German
cryptologists at "B-Dienst" had pierced the Royal Navy codes giving details of
the assembly points and sailing times of convoys, often giving U-boat HQ
between 10 and 20 hours advance warning of enemy intentions. Just as valuable
was German success which between February 1942 and June 1943 frequently enabled
them to read the daily British Admiralty estimate of U-boat dispositions,
though like the Allies, the Germans had to forgo using much information to
avoid the enemy suspecting their success.

Despite the steadily increasing numbers of escort vessels becoming available
for the convoys, increased air support, and technological advances in
anti-submarine warfare, the results of the U-boat war in the second half of
1942 seemed to justify Donitz's hopes. During the last few months of the year,
aided by the diversion of many Allied escorts to support the "Torch" landings
in North Africa, the U-boats were sinking a monthly average of 650,000 tons. If
the vessels sunk by aircraft, mines and such few surface raiders as were still
at large were added to this total, Germany seemed on the verge of achieving the
sinking rate demanded by her experts.

Unfortunately for Donitz's hopes, his planners had seriously underestimated
Allied , particularly American, construction capacity. During 1943 US shipyards
would produce 20 million tons of merchant shipping, ample to replace a total
Allied loss during the previous year of about 7 ½ million tons, overestimated
by the Germans as twice as much. Though there had been many apparently striking
U-boat successes, such as the attack in August on convoy SC94, which had lost
26 ships, and the November assault on SC107, which sank 15 ships, these were
deceptive. In fact, therefore, though not fully appreciated by either side, or
indeed by many modern historians, at the end of the year the U-boats were no
closer to decisive victory. Furthermore, the steadily increasing effectiveness
of Allied anti-submarine measures was hinted at by the less favourable , for
the Germans, sinking ratio, now running at 10 merchant ships for every U-boat
lost.

The Decisive Months

It was apparent to both sides that the first half of 1943 would be decisive.
Donitz began the year believing that the rate of sinkings being achieved by his
crews was slightly outpacing the rate of Allied shipbuilding. His U-boat fleet
had now increased to 400 vessels, of which 200 were operational, and an average
of 100 at sea -10 more than the total which, at the start of the war, Donitz
had argued would have been sufficient to bring decisive victory, although his
estimates then had not allowed for US involvement.

Yet Allied effectiveness was also increasing. There were now over 500 escort
vessels available, sufficient not only to provide stronger close protection for
convoys, but also to allow the formation of "support" or hunter-killer groups,
to reinforce convoys under attack. Equally significant were the on-going
advances in anti-submarine warfare being made by the Allies. During the autumn
of 1942 the increasingly effective air operations against U-boats crossing the
Bay of Biscay in transit to and from their French bases received the welcome
assistance of airborne radar. For a time in the autumn of 1942 the U-boats were
given some protection against this threat by a radar detection device, but in
February 1943 the Allies introduced a new short-wave radar which proved
undetectable until the closing stages of the war.

Although it is unclear whether Donitz had fully grasped the fact, by the
beginning of 1943 there were unmistakable signs that, if it had ever really
existed, the window of opportunity for a decisive German victory in the Battle
of the Atlantic was closing rapidly. The first four months of the year saw the
rapid introduction by the Allies of a whole range of improved anti-submarine
equipment and techniques. As well as improved aircraft-mounted radar, escort
carriers were beginning to prove their worth and surface escorts were being
equipped with radar, high frequency direction finders and improved
anti-submarine weapons such as the "hedgehog" depth charge thrower.

Not only were escorts and aircraft proving more effective, there were also more
of them. Between February and May the number of long-range "Liberator"
anti-submarine aircraft available rose from ten to over sixty. Although the
vast majority of US Navy escort vessels had been diverted by the needs of the
Pacific War, leaving the RN and RCN to perform about 96% of escort duties in
the North Atlantic, increasing the size of convoys had made it possible to
raise the average number of escorts from six to nine, without increasing the
vulnerability of the convoy. Sufficient escorts had also been released to allow
the formation of five British convoy-support groups, the best known being that
operating out of Liverpool under Captain J. F. "Johnny" Walker. They were later
reinforced by a US group. Each group consisted of between five and seven
destroyers and frigates, and three also had an escort carrier. Their role was
to accompany convoys through the mid-Atlantic air gap., where they were most
vulnerable to attack, whilst the presence of an escort carrier helped provide
air support for the entire crossing.

Of equal, though again often underrated significance, was the success by now
consistently being achieved by the Allied radio direction finders. Between July
1942 and May 1943 they managed to divert 105 out of a total of 174 North
Atlantic convoys away from wolf-pack ambushes, and enabled another 23 partially
to avoid such traps. Only 16 ran into large U-boat concentrations, and it was
these which suffered the bulk of losses.

Despite all these favourable portents for the Allies, the first three months of
1943 saw continued notable U-boat successes. They were aided in part by wintry
conditions in the North Atlantic, which made Allied detection less effective,
and also by the introduction of the new U-boat Enigma cypher known to
Bletchley Park as "Shark", which remained unbroken until the end of March.

In February convoy ON 16 lost 14 ships, with the overall merchant ship/U-boat
kill ratio for the month standing at 7:1, a decline from the German high point
of the previous year, but still offering U-boat Command grounds for hope.

The particularly foul weather of March, with convoys and their escorts
straggling through gales, blizzards and hail, saw some of the fiercest battles
of the war. The beginning of the month saw roughly 50 U-boats at sea. Between
7-10 March, convoy SC 121 lost six ships, with 199 men of their crews,
experienced seamen who could be less easily spared than their ships. The next
two convoys, SC 122 and HX 229, were even more savagely mauled by 44 U-boats
from wolf packs Sturmer, Dranger and Raubgraf - the
greatest U-boat concentration achieved in the entire war. A total of 22 ships
of 146,000 tons were lost between March 8-18. But tragic though the loss of
merchant ships and their crewmen was, of ultimately greater significance was
that throughout the entire battle, the U-boats had failed to sink or damage a
single escort vessel, whilst seven of their own number had been damaged, with
two U-boats being sunk later by Allied aircraft on their way back to base.

By April the tide had begun to turn. Though battles were once more fiercely
contested, the Allies lost only half as many merchant ships as in March, whilst
14 U-boats were accounted for.

Feeling the battle slipping away from him, Donitz ordered a supreme effort for
May. The decisive action came in the first week of the month with a concerted
attack on convoy ON 55. After initial German successes, the balance tilted with
the arrival of a Royal Navy support group, assisted by fog which hindered
U-boat operations. On 5/6th May a total of 7 U-boats were sunk compared with 12
merchant ships. It was the beginning of a disastrous trend for Donitz. By the
end of May he had lost from all causes 41 U-boats , more than a quarter of his
operational strength, for barely the same number of Allied merchant vessels.
The losses, as Donitz, admitted, were "intolerable". On May 24th he ordered all
but a token number of U-boats away from the North Atlantic in what was termed
"a temporary shift to areas less endangered". Though no one would be sure of it
for a long time to come, Germany had lost the Battle of the Atlantic.